Macromolecular X-ray crystallography is an essential tool in modern drug discovery and molecular biology. Using X-ray crystallographic techniques, the three-dimensional structures of biological macromolecules, such as proteins, nucleic acids, and their various complexes, can be determined at practically atomic-level resolution from X-ray diffraction data.
One of the first and most important steps in the X-ray crystal structure determination of a target macromolecule is to grow large, well-diffracting crystals of the macromolecule. As the techniques for collecting and analyzing X-ray diffraction data have become more rapid and automated, crystal growth has become a rate-limiting step in the structure determination process.
Vapor diffusion is the most widely used technique for crystallization in modern macromolecular X-ray crystallography. In this technique, a small volume of the macromolecule sample is mixed with an approximately equal volume of a crystallization solution. The resulting drop of liquid (containing macromolecule and dilute crystallization solution) is sealed in a chamber with a much larger reservoir volume of the crystallization solution. The drop is kept separate from the reservoir of crystallization solvent either by hanging the drop from a glass cover slip or by sitting the drop on a pedestal above the level of the solvent in the reservoir. Over time, the crystallization drop and the reservoir solutions equilibrate via vapor diffusion of volatile chemical species. Supersaturating concentrations of the macromolecule are achieved, resulting in crystallization of the macromolecule sample in the drop.
The process of growing biological macromolecule crystals remains, however, a highly empirical process. Macromolecular crystallization is dependent on a host of experimental parameters, including; pH, temperature, the concentration of salts in the crystallization drop, the concentration of the macromolecule to be crystallized, and the concentration of the precipitating agent (of which there are hundreds). In particular, the choice of solute conditions in which to grow crystals continues to be a matter for empirical determination. Testing numerous combinations of variables that affect crystal growth, by means of thousands of crystallization trials, eventually leads to the optimal conditions for crystal growth. Consequently, the ability to rapidly and easily generate many crystallization trials is important in determining the ideal conditions for crystallization. Thus, there is a need for a device that permits the efficient testing of numerous combinations of parameters that affect crystal growth.